Methods and Apparatus to Predict Non-Execution of Conditional Non-branching Instructions

Abstract

Efficient techniques are described for not executing an issued conditional non-branch instruction. A conditional non-branch instruction is identified as being eligible for a prediction, the prediction indicating that the eligible conditional non-branch (ECNB) instruction would not execute. The ECNB instruction executes as a no operation (NOP) instruction in response to the prediction that the ECNB instruction would not execute. A source operand required for the ECNB instruction to execute is not fetched in response to the prediction to not execute.

1 . A method for not executing an issued conditional non-branch instruction, the method comprising:
 identifying a conditional non-branch instruction as being eligible for a prediction, the prediction indicating that the eligible conditional non-branch (ECNB) instruction would not execute; and   executing the ECNB instruction as a no operation (NOP) instruction in response to the prediction that the ECNB instruction would not execute.   
     
     
         2 . The method of  claim 1 , wherein a source operand required for the ECNB instruction to execute is not fetched in response to the prediction. 
     
     
         3 . The method of  claim 1 , wherein a register in a general purpose register file is not reserved to contain the result of the ECNB instruction in response to the prediction. 
     
     
         4 . The method of  claim 1 , further comprising:
 predicting that the ECNB instruction does not execute in response to a disable prediction flag that indicates no prior successful executions of the ECNB instruction occurred during an eligible period on which the prediction was based.   
     
     
         5 . The method of  claim 1 , further comprising:
 recording in a history register whether the ECNB instruction did or did not execute; and   predicting that the next ECNB instruction does not execute in response to the history register indicating at least one prior attempted execution of the ECNB instruction did not execute.   
     
     
         6 . The method of  claim 5 , wherein the at least one prior attempted execution of the ECNB instruction was encountered in a software loop. 
     
     
         7 . The method of  claim 5 , wherein the at least one prior attempted execution of the ECNB instruction was encountered in a pre-specified address range. 
     
     
         8 . The method of  claim 5 , wherein the at least one prior attempted execution of the ECNB instruction was encountered within an identified number of processor cycles. 
     
     
         9 . The method of  claim 1 , further comprising:
 comparing an evaluation criterion with a count value output of an ECNB instruction execution status counter to generate the prediction, wherein the ECNB instruction execution status counter saturates at a first count value indicative of a history of prior attempted executions of the ECNB instruction being strongly not executed.   
     
     
         10 . The method of  claim 9 , further comprising:
 updating the ECNB instruction execution status counter in a first direction to indicate a prior attempted execution of the ECNB instruction conditionally executed; and   updating the ECNB instruction execution status counter in a second direction that is opposite to the first direction to indicate a prior attempted execution of the ECNB instruction conditionally did not execute.   
     
     
         11 . The method of  claim 9 , wherein the evaluation criterion is the first count value. 
     
     
         12 . The method of  claim 9 , wherein the prior attempted executions of the ECNB instruction were encountered in a software loop. 
     
     
         13 . An apparatus for predicting a conditional non-branch instruction would not execute, the apparatus comprising:
 a first circuit for identifying a conditional non-branch instruction as being eligible for a prediction; and   a second circuit for predicting whether or not the eligible conditional non-branch (ECNB) instruction would not execute in response to meeting an evaluation criterion.   
     
     
         14 . The apparatus of  claim 13 , further comprises:
 an operand fetch circuit which does not fetch an operand required for the ECNB instruction to execute in response to the prediction to not execute.   
     
     
         15 . The apparatus of  claim 13 , further comprises:
 a pipeline tracking circuit to track the prediction in pipeline stages following a pipeline stage for predicting; and   an ECNB instruction execution stage circuit which does not execute the ECNB instruction in response to the prediction to not execute.   
     
     
         16 . The apparatus of  claim 13 , further comprising:
 an ECNB instruction execution status counter with a count value output that is compared to the evaluation criterion, wherein the count value is updated in a first direction to indicate an ECNB instruction conditionally executed and saturates at a first count value indicative of a strongly executed history and is updated in a second direction to indicate an ECNB instruction did not execute and saturates at a second count value indicative of a strongly not executed history.   
     
     
         17 . The apparatus of  claim 16 , wherein the evaluation criterion is the second count value. 
     
     
         18 . The apparatus of  claim 13 , wherein the evaluation criterion is a disable prediction flag in an non-active state, wherein the non-active state of the disable prediction flag indicates prediction is enabled, wherein the disable prediction flag is set to a disable state if the ECNB instruction is ever determined to have conditionally executed in a software loop associated with the ECNB instruction. 
     
     
         19 . A method for predicting a conditional non-branch instruction would not execute, the method comprising:
 identifying a conditional non-branch instruction that is eligible for predicting whether it will or will not execute; and   predicting that the eligible conditional non-branch (ECNB) instruction will not execute in response to meeting an evaluation criterion.   
     
     
         20 . The method of  claim 19 , wherein a source operand required for the ECNB instruction to execute is not fetched in response to meeting the evaluation criterion. 
     
     
         21 . The method of  claim 19 , wherein the ECNB instruction is executed as a no operation (NOP) instruction in response to meeting the evaluation criterion. 
     
     
         22 . The method of  claim 19 , wherein meeting the evaluation criterion comprises:
 recording a history of execution status of previous attempted executions of the ECNB instructions encountered within a software loop; and   comparing the history with the evaluation criterion to indicate whether the evaluation criterion has been met.